p53 deficiency does not affect the accumulation of point mutations in a transgene target.

Abstract

DNA repair is required by organisms to prevent the accumulation of mutations and to maintain the integrity of genetic information. Mammalian cells that have been treated with agents that damage DNA have an increase in p53 levels, a p53-dependent arrest at G1 in the cell cycle, and a p53-dependent apoptotic response. It has been hypothesized that this block in cell cycle progression is necessary to allow time for DNA repair or to direct the damaged cell to an apoptotic pathway. This hypothesis predicts that p53-deficient cells would have an abnormal apoptotic response and exhibit a "mutator" phenotype. Using a sensitive assay for the accumulation of point mutations, small deletions, and insertions, we have directly tested whether p53-deflcient cells exhibit an increased frequency of mutation before and after exposure to DNA-damaging agents. We report that wild-type and p53-deficient fibroblasts, thymocytes, and tumor tissue have indistinguishable rates of point mutation accumulation in a transgenic lacI target gene. These results suggest that the role of p53 in G1 checkpoint control and tumor suppression does not affect the accumulation of point mutations.

Dissociation of p53-mediated suppression of homologous recombination from G1/S cell cycle checkpoint control

“…Radiation-induced DNA damage may resemble DNA ends that are present during recombinational exchanges. However, we and others have not found p53-dependent apoptosis after irradiation to play a role in non-transformed MEF (Sands et al, 1995;Mekeel et al, 1997). Furthermore, apoptosis was unlikely to account for recombination di erences seen between 10.1 and 10.1/Val5 cells (Table 1), since growth characteristics, transfection frequencies and plating e ciencies for these cell lines have not revealed signi®cant di erences (data not shown).…”

Absence of p53 permits propagation of mutant cells following genotoxic damage

“…We therefore conclude that despite the lack of data on p53 +/+ wild type mutant frequency, the clonogenic p53 7/7 cells surviving 4 Gy have an hprt mutant frequency that is probably in the normal range for this level of radiation exposure and that p53 de®ciency does not lead to hypermutability following DNA damage at least in this assay system. Nishino et al (1995) and Sands et al (1995) using a bacterial lacI transgene as a marker for calculating mutation frequency (in Big Blue6p53 7/7 mice) also reported normal frequencies for both spontaneous and genotoxic damage-induced mutants. However, there are several reasons why the bacterial lacI gene might not provide an appropriate read out: in particular large deletions (as found in the hprt gene) would probably be missed because of size constraints on the packaging into phage particles involved in the assay and deletion of the selection marker along with lacI would lead to an underestimation of mutants.…”

“…However, there are several reasons why the bacterial lacI gene might not provide an appropriate read out: in particular large deletions (as found in the hprt gene) would probably be missed because of size constraints on the packaging into phage particles involved in the assay and deletion of the selection marker along with lacI would lead to an underestimation of mutants. These reservations are reinforced by the ®nding that an endogenous expressed gene Dlb-1 had a much higher level of X-ray-induced mutations (mostly involving large deletions) than a bacterial lacI transgene (Sands et al, 1995). Our observations using hprt as a marker gene for deletion mutations contrasts with the high (100 ± 4506) spontaneous hypermutability of the hprt gene in colon cancer cells with genome instability or`replication error' phenotypes (Bhattacharyya et al, 1995;Eshleman et al, 1995) and may appear to be at odds with the earlier observations of very high rates (*10 74 ) of ampli®cation in a drug-resistant gene in p53 7/7 cells (Yin et al, 1992;Bou er et al, 1995).…”

Li-Fraumeni syndrome – a molecular and clinical review

“…This sensitivity is associated with an increase in radiation-induced double-strand chromosomal breaks . Transgenic mice carrying mutant TP53 genes show cellular radioresistance in haematopoietic cells (Lee and Bernstein, 1993 (Nishino et al, 1995;Sands et al, 1995).…”